1. Field of the Invention
The present invention relates to ceramic multilayer substrates and methods for manufacturing the same, and more particularly, to a ceramic multilayer substrate having a ceramic chip electronic component inside and a method for manufacturing the same.
2. Description of the Related Art
Japanese Examined Patent Application Publication No. 06-32378 (Patent Document 1) discloses an electronic component-embedding multilayer ceramic substrate and Japanese Unexamined Patent Application Publication No. 2002-084067 (Patent Document 2) discloses a multilayer ceramic substrate and a method for manufacturing the same.
The electronic component-embedding multilayer ceramic substrate disclosed in Patent Document 1 includes a multilayer ceramic substrate, a ceramic chip electronic component disposed in a recess or a space defined by a through hole in the multilayer ceramic substrate, and conductors wiring the chip electronic component and disposed between layers or in spaces of the multilayer ceramic substrate. Since the chip electronic component is disposed in a space of the multilayer ceramic substrate, the resulting multilayer ceramic substrate can have a desired shape so as to ensure flatness.
In the multilayer ceramic substrate and its manufacturing method disclosed in Patent Document 2, functional elements, such as a capacitor element, an inductor element, and a resistor element, are prepared in advance using plate-shaped sintered compact plates (corresponding to the ceramic chip electronic component) that are prepared by previously firing the ceramic functional elements, and these functional elements are embedded in a green multilayer composite. The green multilayer composite includes a green base layer, a constraining layer including a sintering-resistant material, and wiring conductors. When the green multilayer composite is fired, the constraining layer constrains the shrinkage of the green base layer in the main surface direction. Such a constrained sintering process using the constraining layer allows the green multilayer composite to be fired with the embedded ceramic functional elements without problems, and prevents mutual diffusion between the constituents of the ceramic functional elements and the green base layer. Thus, the functional elements maintain their properties even after firing.
Unfortunately, the multilayer ceramic substrates disclosed in Patent Documents 1 and 2 are manufactured by firing a stack of ceramic green sheets having a ceramic chip electronic component disposed therein. Consequently, the ceramic chip electronic component embedded in the fired multilayer ceramic substrate may be cracked or broken. This occurs in the constrained sintering process using a constraining layer. In addition, since the multilayer ceramic substrate is fired with the ceramic chip electronic component in close contact with the ceramic green sheets, it is difficult to prevent the mutual diffusion between the constituents of the ceramic chip electronic component and ceramic layers. Even the technique disclosed in Patent Document 2 may degrade the properties of the resulting chip electronic component.
Furthermore, in a multilayer ceramic substrate having a recess or through hole defining a cavity into which the ceramic chip electronic component is disposed, as disclosed in Patent Document 1, the strength of the substrate around the cavity may be significantly degraded.